Active magnetic bearings (AMBs) have recently emerged as an attractive technology for controlling the position of a wide range of rotating machinery shafts, particularly turbomachine rotors.
Turbomachines such as compact compressors combine a pressurized, high-speed motor and magnetic bearing system with the compressor in a single, hermetically sealed motor-compressor module. FIG. 1 illustrates a cross-sectional view of a conventional compact compressor 100. The conventional compact compressor 100 has a compressor end 102 and motor end 104. A pressure casing 106 hermetically seals the compressor and the motor. As illustrated in FIG. 1, a primary bearing system including, for example, active magnetic bearings (AMBs) 108 may be employed at various locations along the shaft of the conventional compact compressor 100. AMBs may typically be located at the ends of the shaft and, depending upon the length of the shaft, at one or more locations between the ends of the shaft. In addition to the primary bearing system, current designs may require an auxiliary bearing system to allow the rotor system to run non-destructively for some period if a failure of the primary bearing system occurs.
FIG. 2 illustrates a cross-sectional view of a portion of a rotating machine utilizing a conventional auxiliary bearing system. FIG. 2 also illustrates a primary bearing system used in conjunction with the conventional auxiliary bearing system. In FIG. 2, a shaft 202 of the rotating machine is supported by the primary bearing system including at least a pair of radial active magnetic bearings 204 (only one shown) and a thrust active magnetic bearing 206 during normal operation of the rotating machine. When the primary bearing system fails, the shaft 202 is supported by the conventional auxiliary bearing system including at least a pair of auxiliary bearings 208.
During operation, auxiliary bearings control the radial position and the axial position of the rotor system (for example, the shaft and/or other rotating components) while static and dynamic radial and thrust forces act on the rotor system. The auxiliary bearings are typically inactive while the rotor system of the rotating machine is supported by the primary bearing system during normal operating conditions. This is achieved by providing some clearance (both axial and radial) between the interfacing surfaces of the auxiliary bearings and the rotor system. When the primary bearing system fails, the shaft de-levitates and the rotor “drops” radially (for a horizontal axis rotor system) onto the auxiliary bearing system. Appropriate portions of the auxiliary bearing system accelerate to rotor rotational speed and take over the bearing function from the failed primary bearing system. A significant problem of such a system is controlling the shaft position when the rotor “drops” on the auxiliary bearing system. Some conventional auxiliary bearing systems can become unstable during the transient event, resulting in damage to the rotating machine.
There is a need, therefore, for an auxiliary bearing system that stably supports the rotating shaft when the primary bearing system fails and further minimizes damage to the rotating machine.